Re: ATM astigmatism with a sling?

["John Sherman" <shermj@netzero.net>]

Hello, everybody,

This is going to be pretty long, in case you want to delete it now...

OK, the point here is not that I'm trashing Galaxy Optics, it's about the sling.
I have been very curious as to the magnitude of the sling problem. Has anyone
actually ever SEEN this astigmatism/potato-chipping in their own scope?
Supposedly the reason this became such a big issue a while back is that Hudek
did some comparison tests with different cells on his interferometer.
Supposedly, he could see the difference in the orientation of the mirror (the
grain thing), as well as the difference when less than 50% of the mirror is
supported (the mirror is pinched from the sides when the sling touches a full
50% or more). I'm thinking that these issues are only evident when you have
near-perfect optics and collimation, since I can't see them on my own scope. If
those of you with very thin mirrors cannot see the difference upon
experimentation, then it is probably not worth worrying about any further.
I collimate my scope at 50 or so degrees elevation. When I'm done, I can move up
and down in altitude and see the thing go slightly out of collimation (with a
Cheshire) due to the stretching of the sling (I have a 2" nylon sling & 2.125"
mirror). I have seen this in every sling mounted mirror I've played with.
One problem with trying to evaluate the sling setup is that it only works to
it's full potential near the horizon, where the seeing is normally bad. And if
the seeing is a lot worse than the astigmatism, why worry about it? Another
thing, since we know the sling stretches, are all of us taking into
consideration the stickiness of our pads?? If your pads are sticky, they will be
pulling up on the back of the mirror while the front sags down into the sling.
That has to cause some problems. Best to have slippery pads on your
triangles....

Now,
I have been very disappointed in the performance of my scope for over three
years. That's why I keep investigating all options, trying to find what I did
wrong when building it. I have rebuilt my cell. I have collimated the &^%$# out
of it. I have used it next to other large aperture scopes in excellent seeing.
It always looks like the problem is a poor primary. My secondary also came from
Galaxy, the interferogram says 0.095 wave/front. Mighty pleasing for a big 4"
flat. And I have never seen a reason to doubt that reading. I have tested my
scope with lots of eyepieces, from Naglers to Plossls, there is no variation
there.

If I had gotten a great mirror from him, I'd be singing praises. Yes I know he
has a great reputation. I'm told that reputation was built up by his former
employee, who had a gift at parabolizing mirrors. Note the word "former". There
are many
people with older Galaxy mirrors that have wonderful figures (they have Jerry
Wilkinson's initials carved into the back). I have looked
through lots of them. That's why I bought from Galaxy, even though his price is
higher. But now that good reputation is going down the toilet. I have asked this
list if there's a place 'round Los Angeles where I can have an interferometer
test. There was no reply. (I want an interferometer test because it's less
ambiguous than a Foucault)  Hudek sent me an interferogram the first time
showing
a P-V error of 0.4 wave/front---nowhere near diffraction limited. After
refiguring the mirror, he refused to send me another interferogram.
Why is that?
(I called him and asked for it)

Apparently, Hudek does not have a gift for parabolizing. If you buy from him
now, you're taking a big chance. Maybe you'll get a good one, probably not.
Several people
have said the star test looks like over-correction. If anyone around LA wants to
do a Foucault or other test, I'll bring the mirror to you (I'm not willing to
ship it). (email me off-list)  I could build a tester, but I don't have the
experience with lots of mirrors so that I could draw a definitive conclusion.
This is not something I jumped hastily to. I've been playing with this
problem for over three years. I will let any knowledgable person examine my
scope and try to tell me what the problem is.
Indeed, I am desperate to find a solution. All I want is high-resolution, so I
can observe in happiness.

I thank all those who replyed, including those I quote below (in the order
received). Everyone gave good suggestions/questions, most of which I have
already tried to resolve in the past.

> What does your test indicate for figure?
>Is the mirror thermally
>stable when you do your test? Also what is your focal ratio?
>Is it possible that you are mixing spherical abberation
>and coma abberation?
> - Mike -
The star test goes like this: Outside focus the rings look reasonable. As you
approach focus, and before you get there, a fuzzy blur with spikes all around
the edge appears. When I reach the place where the "outside" rings would reach a
focus, the fuzzy blur has grown in size and strength. Inside focus is so bad
that rings cannot be discerned at all, just a big blur with spikes. The area of
"best" focus is a big mushy area, typical of spherical aberration. It does not
have a spot where it "snaps" into best focus. I have an f/4.5, which does get to
a fairly good thermal equilibrium. I have played with my ventilation system
quite a bit. I'm convinced I have good collimation, and I know coma is not an
on-axis problem, as is spherical aberration.

>Otherwise the best that I can do is to hazard a
>guess that you are overcorrected in the outer zones.
>Mel
Remarkable! That is the same conclusion of most people who have actually seen
the scope!

>What are your qualifications to do these tests?
>He has a very fine reputation (which you are attempting
>to trash) and I think that there just has to be something
>else going on here.
>Ken
My qualifications are all the very fine scopes I have looked through in the last
fifteen years. Other than that, now you may know why I'm trying to get an
interferometer test. Once you see what very fine optics will show you, mushy
optics are never good enough. Even my little 8" easily outperforms my 22"
(resolution wise).

>was disappointed, and gave the usual 'blame the mirror'
>off the cuff comment.  Is this correct, John?
>Mel
Three years ago it was correct, since then I've done lots of investigating.

>If you are disappointed with the scope, would you like to sell it?
>-Ralph
YES!!  Just as soon as I can get another big scope to look thru. As bad as the
figure is, the 22" aperture is wonderful. I can't live without aperture.

>in 20 years of working with large thin
>blanks, this is the first time I have ever heard of the phrase
>'grain of pyrex'.  As far as I am concerned, the
>annealling had better remove any overall orientation of strain.
>Mel
It was John Hudek himself who explained all this to me about the grain in his
glass. At this point, I give very little credence to anything he has to say. As
I said, I tested for it, but if something is there it is smaller than my other
problems.

>There is no chance any of my last four
>mirrors will ever be used to its potential in Georgia
>Frank
I don't agree. If your seeing is a six out of ten, I think you will find you can
see the difference. A sharp optic will cut thru mediocre seeing better than a
bad optic.

>I'm kind of curious by now to know where the bug is....
>I hope it's OK my forwarding your tip.
>Nils Olof
Yes it is! And thank you for the support! I don't know where El Camino College
is, but if it's around here and Dave has the experience, I will gladly bring my
mirror/scope to him for testing. (email me off-list)

>This would indicate to me that he does care about his reputation.
>Ken
Well, when I called and emailed him he decided not to help me.


You ATM'ers are among the very best people on our planet! Thank you all,

John
















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